WorldWide Drilling Resource

17 MAY 2021 WorldWide Drilling Resource ® Top-of-the-World Tunnel Compiled by Carol C. Schimpf, Editorial Assistant, WorldWide Drilling Resource ® Embedded under more than a mile of snow-covered rock at the top of the Himalayan Mountains in the eastern Pir Panjal Range, India, the Atal Tunnel, formerly called the Rohtang Tunnel, was opened to traffic in October 2020. This tunnel bypass- es one of the world’s most treacherous roads, on which dozens of people died each year trying to cross through the high-alti- tude pass. In planning for decades and under construction for almost another decade, the Atal Tunnel promises to change the lives of isolated village residents for the better. The Kullu Valley is connected through the Rohtang Pass to the Lahaul and Spiti Valleys. At an altitude of 13,000 feet, the dangerous highway was open for only four months of the year due to unpredictable snowstorms and blizzards. The pass was named Rohtang, derived from the Persian and Farsi words Ruh and Tang, which combined mean “a pile of corpses”. Almost 20,000 people in the valleys were cut off from the rest of India, isolated from November to May, and forced to fly out on hel- icopters, weather permitting, if there were medical emergencies. Around 3500 to 4000 people were airlifted each year. Virender Sharma, the chief government official in Kyelang, the main town of the Lahaul Valley said, “In the winter, there is no light. No vegetables. No mail. Nothing to do in the evening. If there is an emergency, you are practically at the mercy of God.” Constructing an approximately five-mile tunnel in such a harsh, high-altitude environment was a daunting task. Complex topography and geologies, including peaks of 17,000 feet, presented enormous challenges to geological work, tunnelling, as well as mechanical and electrical installations. The Himalayas are full of geological surprises, including faults, folds, and shear zones, revealing their presence due to continuing tectonic movement. These features cannot be mapped out during the geo- logical investigation prior to starting a project. They are hidden in such a way to make mapping impossible unless a project starts and real-time excavation begins. Not knowing exactly what kind of rock was inside the mountain, Thomas Riedel, a German contractor remarked, “That is where we will find problems.” Highly overburdened features of the material increased the problems, creating uncertainty in designing a particular support system and demanding a design-as-you-go methodology for the entire tunnel length. The New Austrian Tunneling Method (NATM), based on deformation monitoring data to evaluate quantity and quality of the support system, proved to be an appropriate tool for tunnelling in this environment. Once construction began, the NATM was driven to its limits. Crumbly-textured soils instead of blast rock, excess water ingress, high rock pressure with extreme deformations, and rapid changes in rock formations were major challenges to overcome. The tunnel was driven from two sides by the painstaking method of drilling and blasting, rather than using tunnel boring machines. Advanced techniques, including forepoling, lattice girders, pipe roofing, pregrouting, and chemical grouting, were applied. The horseshoe-shaped cross-section with an excavation diameter of more than 42 feet carries two lanes of vehicle traffic, two footpaths, and an emergency egress beneath the carriageway. During the nearly ten years of construction, Snowy Mountains Engineering Corporation (SMEC) provided detailed engineering and advisory services. Their innovation and technical skills allowed contractors to contend with poor rock-flowing conditions, avalanches, blizzards, and other challenges. Chief Engineer for the $230 million project, P.K. Mahajan, proudly commented on the tunnel completion as “an engineering marvel for the whole nation.” Designed to handle up to 3000 vehicles per day in any weather conditions at a maximum vehicular speed of nearly 50 miles per hour, the two-way traffic facilitates travel between the remote regions and the rest of India, effectively reducing travel distance by about 29 miles and travel times by up to four hours. The better transportation route is expected to improve the economy by allowing residents to sell their agricultural produce in towns and by opening the remote towns to more tourists. It will also provide a winter link to families and hometowns for those in remote regions. Atal Tunnel now provides safe, all- weather travel in the northern regions of India, turning a once dangerous journey at the top of the world into a modernized pathway connecting a nation’s people. Constructing the Atal Tunnel bypassing Rohtang Pass. Photo courtesy of Snowy Mountains Engineering Corporation. C&G